Pharmacodynamics of 5-amino levulenic acid (ALA) conversion to a photoactive protoporphyrin IX (PpIX) in organ tissues of normal cats and in naturally occurring squamous cell carcinoma of the skin of a single cat was investigated. Pre and 4 hour post ALA administration (100 mg/kg, 200 mg/kg and 400 mg/kg) tissue samples were collected. Frozen sections were imaged by phase contrast and fluorescence emission, then dried, H&E stained, and re-examined by phase contrast. Fluorescence presence and relative intensity in sections was used to evaluate location and relative magnitude of PpIX presence in the tissues. Structures of normal skin (epidermis, hair bulbs and sebaceous glands) were found to have post treatment accumulations of PpIX, as did the epithelial surfaces of other organs. Fluorescence intensity in tumor was more than 3 times that observed in hair bulbs and sebaceous glands, 5 times more than observed in epidermis and 20 times more than observed in dermis , ar rector pili muscle and adipose tissue.